1. Field of the Invention
The present invention relates to fuse housing assemblies, and more particularly, to bayonet-type fuse housing assemblies that receive a fuse for a transformer circuit.
2. Description of the Related Art
In many instances, it is necessary to protect a transformer or an electric circuit from excessive current. For example, a current limiting fuse is typically in series with the transformer circuit and is configured to trip or break at only very high current levels. A bayonet-type fuse assembly is also connected in series with the transformer circuit and is generally configured to trip at current levels smaller than that of the current limiting fuse, although there may be some overlap between the current trip levels of the two current limiting devices. An example of such a bayonet-type fuse assembly is illustrated in U.S. Pat. No. 5,227,758, the entire disclosure of which is hereby incorporated by reference. When current exceeds a specific amperage, the fuse in the bayonet-type fuse assembly located in the transformer body will melt or vaporize and thus break or open the transformer circuit.
Because of the high voltages (7.2, 14.4, 23, 24.9 and 34 KV) associated with transformers, fuses that break the circuit of the transformer are typically located in the transformer and under a dielectric liquid, such as mineral oil, within the transformer. The dielectric liquid provides arc quenching during fuse operation and also prevents arcing between the fuse contacts and the walls of the transformer or other items within the transformer.
In a conventional bayonet-type fuse holder housing for receiving a fuse, a cylindrical housing of electrically insulating material protrudes into the transformer at a downward angle such that a portion of the cylindrical housing is located under the oil located within the transformer. A portion of the cylindrical housing is also located external of the transformer, and protrudes from the transformer. A bayonet-type fuse holder having an attached cartridge with fuse link may be inserted into the interior of the cylindrical housing such that the fuse is located near the most distal end of the housing under the oil and is in contact with a pair of contacts. The portion of the fuse housing located within the transformer body has openings that allow the fuse to be directly exposed to the oil. The bayonet-type holder and cartridge fuse are removable from the fuse holder housing by simply pulling on an end of the bayonet-type fuse holder that protrudes from the portion of the fuse housing that is located external of the transformer body.
During operation of the above-described bayonet-type fuse assembly, it is necessary that the fuse be entirely submerged within the oil such that when operating conditions occur and the fuse operates or melts, any electrical arc between the conductive opposite ends of the fuse will be quickly extinguished by the dielectric oil. Thus, the dielectric oil helps extinguish the arc. The oil also cools equipment within the transformer.
After the fuse has operated, it is necessary that a new fuse be inserted into the fuse holder housing in order to reenergize the electrical circuit to permit the transformer to operate. However, before a new fuse may be inserted into the fuse holder housing, it is necessary that the operated fuse held by the fuse holder be removed from the fuse holder housing such that the operated fuse may be replaced with a new fuse.
To replace the open fuse, it is necessary that an operator remove the bayonet-type fuse holder completely from the fuse holder housing. However, most, if not all, transformer tanks are sealed from the exterior environment to prevent water, moisture, dirt and other contaminants from entering the interior and degrading the dielectric fluid (oil) of the transformer. Because the transformer tank is sealed from the surrounding environment, it is common that the transformer tank becomes pressurized. This increase in pressure primarily occurs because the oil within the transformer tank expands when its temperature is increased. Expansion will occur when the transformer is exposed to sun in warm climates and when the transformer is operating under high load conditions. Thus, it is apparent that the oil level within the transformer tank varies according to operating conditions. The pressure within the tank may also increase because of fuse operation. Temperature variations can cause the oil level to vary approximately 5 inches on a 72 inch high transformer tank.
If the transformer tank is pressurized, upon removal of the bayonet-type fuse from the bayonet-type fuse housing, a seal will be broken, relieving the pressure inside the tank to possibly cause dielectric oil from the interior of the transformer to travel up through the bayonet-type fuse holder housing due to the differential in pressure between the tank and the surrounding atmosphere. In fact, a high velocity liquid stream of hot oil may jet out of the portion of the bayonet fuse holder housing that protrudes from the transformer tank. That is, when the bayonet fuse holder is removed from the fuse holder housing, the tank is no longer sealed and the pressure within the tank will equalize with atmospheric pressure via the fuse holder housing that communicates the interior of the transformer with the exterior environment.
The oil that flows out of the fuse holder housing may be at a high temperature and may injure the operator who has removed the fuse. Under normal operating conditions the oil may be between 90-100.degree. C. (194.degree. F.-212.degree. F.), and under extreme conditions, such as during hot summer days in warm climates and when the transformer has been under full load, the oil may be approximately 120.degree. C. (248.degree. F.).
Because it is necessary that the fuse is located completely under the oil, at all operating temperatures, it is undesirable that none of the oil exit the transformer. If the oil level becomes too low in the transformer tank, the upper contact of the fuse may be exposed to air, causing danger to personnel removing the fuse as an arc between the upper contact and the transformer wall can result.
There have been various attempts to address the above problems. For example, conventional transformer housings attempt to address this problem by providing a pressure relief valve that actuates automatically upon an increase in pressure within the transformer tank. For example, known pressure relief valves typically will bleed at approximately eight pounds per square inch (PSI). Nevertheless, it is common that the transformer is pressurized between atmospheric pressure and the bleed pressure. This pressure is sufficient to cause the dielectric oil to flow from the fuse holder housing when the fuse holder is removed. In some instances, oil has ejected from the housing for approximately 20 seconds while the pressure within the tank equalizes to atmospheric pressure. Some transformer tanks do not include pressure relief valves.
Other transformer tanks include a manually actuatable pressure relief valve that may be manually actuated by an operator to bleed all pressure within the transformer housing prior to removal of the bayonet-type fuse holder from the fuse holder housing. Nevertheless, operators are reluctant to approach transformer tanks that house live high voltage equipment. Operators also forget to actuate the pressure relief mechanism to bleed any pressure within the transformer such that when the fuse holder is removed from the fuse holder housing, oil from within the interior of the transformer is forced out of the fuse holder housing. After the pressure relief valve has been actuated, the pressure within the tank may increase slightly before an operator has the opportunity to remove the fuse holder from the fuse holder housing. This slight increase in pressure may be sufficient to also cause the dielectric oil to flow out of the fuse holder housing.
Other attempts to prevent the oil from exiting the fuse holder housing include a flapper valve that seals a portion of the fuse holder housing exterior of the tank from a portion of the fuse holder housing within the tank such that when the fuse holder is removed, the flapper valve closes to form a seal and the oil cannot escape from the fuse housing. Such a flapper valve is disclosed in U.S. Pat. No. 5,204,654, the entire disclosure of which is hereby incorporated by reference.
However, the transformer tank is still pressurized with use of the flapper valve device alone, and it is necessary that the operator quickly remove the fuse holder from the fuse housing to prevent the dielectric oil from flowing out of the fuse housing. If the operator does not remove the fuse holder in one quick movement, the oil will flow out of the housing until the operator has sufficiently removed the fuse holder from the housing such that the flapper valve may close. Likewise, when the operator inserts the fuse holder back into the fuse holder housing, because the tank is still pressurized, the oil may again flow out of the housing after the flapper valve has been opened and until the time when the operator has sealed the fuse holder housing with the fuse holder.
Other bayonet-type fuse holder housings include a hole located in the housing that communicates the interior of the tank with the interior of the housing. The hole is located in the portion of the fuse holder housing that is located within the transformer tank and is adjacent to the area of the housing that mounts to the wall of the transformer tank. Thus, when the fuse holder is removed from the fuse holder housing, and the seal between the fuse holder and the fuse holder housing is broken, any pressurized gas within the housing will vent through the hole to atmosphere--so long as the oil level is below the hole.
However, this configuration is problematic because pressurized gas within the housing that is rapidly venting through the hole tends to draw oil from within the transformer tank such that a spray of gas and oil is ejected from the fuse holder housing. This is particularly problematic when the oil commonly expands due to an increase in temperature such that it is directly below the hole in the cylindrical housing. Moreover, the oil level may move above the hole in the housing. Although transformer tanks are intended to be level, on occasion the ground on which the transformer is positioned may settle or move causing the transformer to tip. The transformer may also be improperly positioned such that the hole is located below the oil level in the transformer tank. A tilt angle of 2 or 3 degrees can shift the oil level one inch in some transformer tanks. The oil level may also move above the hole because it expands due to a temperature increase. In this case, a high velocity stream of oil shoots out of the housing when the fuse holder is removed. This is especially problematic because operators and maintenance people often overfill the transformer tank with the dielectric oil.
Accordingly, it is apparent that conventional fuse holder housings that attempt to prevent oil from exiting the transformer through the face holder housing are not easily adaptable to changing conditions. The above-described constraints and problems associated with removing a fuse holder from a fuse holder housing mounted to a transformer tank has created a need for a solution.